The heart is located roughly in the center of the chest cavity. It is covered by a protective membrane, the pericardium.

Deoxygenated blood from the body enters the right atrium.

It flows through the tricuspid valve into the right ventricle. The term tricuspid refers to the three flaps of tissue that make up the valve.

Contraction of the ventricle then closes the tricuspid valve and forces open the pulmonary valve.

Blood flows into the pulmonary artery.

This branches immediately, carrying blood to the right and left lungs.

Here the blood gives up carbon dioxide and takes on a fresh supply of oxygen [More].

The capillary beds of the lungs are drained by venules that are the tributaries of the pulmonary veins.

Four pulmonary veins, two draining each lung, carry oxygenated blood to the left atrium of the heart

Below: the human heart, with a schematic view of the pathway of blood through the lungs and internal organs. Oxygenated blood is shown in red; deoxygenated blood in blue. Note that the blood draining the stomach, spleen, and intestines passes through the liver before it is returned to the heart. Here surplus or harmful materials picked up from those organs can be removed before the blood returns to the general circulation. [Graphic of this hepatic portal system]

Blood flows through the mitral valve (also known as the bicuspid valve) into the left ventricle.

Contraction of the ventricle closes the mitral valve and opens the aortic valve at the entrance to the aorta.

The first branches from the aorta occur just beyond the aortic valve still within the heart.

Two openings lead to the right and left coronary arteries, which supply blood to the heart itself.

Although the coronary arteries arise within the heart, they pass directly out to the surface of the heart and extend down across it. They supply blood to the network of capillaries that penetrate every portion of the heart.

The capillaries drain into two coronary veins that empty into the right atrium.

The coronary arteries arise at the point of maximum blood pressure in the circulatory system. Over the course of time, the arterial walls are apt to lose elasticity, which limits the amount of blood that can surge through them and hence limits the supply of oxygen to the heart. This condition is known as arteriosclerosis.

Fatty deposits, called plaque, may accumulate on the interior surface of the coronary arteries. This is particularly common in people who have high levels of cholesterol in their blood. Plaque deposits reduce the bore of the coronary arteries and thus the amount of blood they can carry.

Atherosclerosis (usually along with arteriosclerosis) may

so limit the blood supply to the heart that during times of stress the heart muscle is so deprived of oxygen that the pain of angina is created.

trigger the formation of a clot causing a coronary thrombosis. This stops the flow of blood through the vessel and the capillary network it supplies causing a heart attack.
The portion of the heart muscle deprived of oxygen dies quickly of oxygen starvation. If the area is not too large, the undamaged part of the heart can, in time, compensate for the damage.